bugfix: fix bug for mkdir command

[bryantanwz]

In Tutorial 6, the following error message occurs when trying to run the tutorial more than once:

Cell In[10], line 14
11 index = femr.index.PatientIndex(dataset, num_proc=4)
12 main_split = femr.splits.generate_hash_split(index.get_patient_ids(), 97, frac_test=0.15)
---> 14 os.mkdir(os.path.join(TARGET_DIR, 'motor_model'))
15 # Note that we want to save this to the target directory since this is important information
17 main_split.save_to_csv(os.path.join(TARGET_DIR, "motor_model", "main_split.csv"))

FileExistsError: [Errno 17] File exists: 'trash/tutorial_6/motor_model'

The error occurs because the directory 'trash/tutorial_6/motor_model' already exists when the code tries to create it. To fix this, use os.makedirs() instead of os.mkdir() with a exist_ok=True parameter so a FileExistsError is not raised if the directory already exists. This allows for the tutorial code to be run more than once.